Hey Jazz, thanks for taking the time man, appreciate that, and I do feel much reassured :D

That's what I thought about the 0.15mm, I was less sure about the 0.35mm wobble - but I guess that is just 0.15mm deflection on either side of some theoretical centre line...

I do plan to tension them, and they have threads on both ends for that reason.
Actual details of how I mount them need working out though: the steel brackets on either side are substantial enough to put a serious bit of tension on them, and I don't want to pull them so much that I lose accuracy on the screw's pitch Any idea what kind of force I should be aiming for with these 32mm screws if tightening the M25 x 1.5 nuts with a torque wrench?
I want to be absolutely sure that the tensioning nuts are pushing onto a perpendicular surface - I know it's not ATM, even by eye, and would probably 'pull' a bend into the screw if torqued up the way they are. I was planning to mount them loosely and use metalised epoxy to create a perfectly perpendicular & flat surface to tighten against...

Well, thanks again, I guess I better get on and do some lathe work on the rotating ball nut spindles, so I can actually mount them ;)